Add Erlang R15 support

Hi - great plugin.

This small extension adds support for Erlang R15 (and greater) style stack traces.  It might be possible to write an extractor for Erlang R14 (which didn't include file and line number info) but that's currently beyond my understanding for vimscript.  

Since I write mostly R14 Erlang for work, I may try to work on it anyway though.